The present invention relates generally to a scramjet combustor for a supersonic flight vehicle and more particularly to a scramjet combustor having a two-part, aft-facing step for improved combustor performance.
Although the theory of scramjet engines has been well known for many years, and although supersonic combustors have been tested in the laboratory, no scramjet engine has been reported to have flown successfully. Recent advances in technology, such as in high temperature materials, have made scramjet engines ready for implementation in the next generation of high speed aircraft. Such aircraft will be capable of flying at hypersonic speeds (i.e., speeds having Mach numbers greater than about 5.5).
Hypersonic flight vehicles have been proposed which incorporate scramjet engines to achieve high Mach numbers. Once such a vehicle has achieved a sufficient speed by some other propulsive means. (which may include a turbojet engine), a scramjet engine will take over to propel the aircraft to high Mach numbers (typically between Mach 6 and Mach 20). Such high Mach numbers cannot be achieved by any other known type of air-breathing engine.
A typical scramjet engine includes a combustor having a chamber, wherein a fuel-air mixture moving at supersonic speed is burned, and having at least one fuel injector which directs supersonically-moving fuel (such as pressurized hydrogen) into the chamber. The engine also includes an air inlet, which delivers compressed supersonically-moving air to the combustor chamber, and further includes an exhaust nozzle, which channels the burning gases out of the combustor chamber to help produce engine thrust. The fuel injector discharge orifices are the openings in the combustor chamber to which fuel is delivered by a fuel system which may include tanks, pumps, and conduits. In the case of hydrogen fuel, the fuel-air mixture in the combustor chamber will have a high enough temperature and pressure to auto-ignite.
Known scramjet combustor designs include combustor walls having an aft-facing step and include an angled fuel injector located at the top of the step or a transversely-directed fuel injector located at the bottom of the step. In all cases, known fuel injectors appear to be round conduits. "Angled" fuel injection means the injected fuel is not parallel or perpendicular (transverse) to the generally longitudinally-moving air. It is known in the art that as the flight Mach number increases above about Mach 10-12, a substantial and increasing portion of the engine thrust comes from the discharge of the pressurized fuel from the angled fuel injectors and not from the burning of that fuel. It is also known that angled or transverse fuel injection promotes significantly better fuel penetration into the airstream and better fuel-air mixing than can be achieved from axial injection. Consequently, to achieve adequate fuel penetration across the height of the combustion chamber with axial injection, it is common practice to utilize some type of fuel injector which protrudes into the airstream. Such practice produces large thrust losses and requires increased fuel cooling requirements. It is further known that an aft-facing step stabilizes combustion by providing a "shield" which keeps combustion-induced pressure rises from propagating upstream, which can otherwise occur due to varying inlet conditions and perturbations in fuel flow and which can lead to engine shutdown (also called inlet "unstart"). What is needed is a scramjet combustor having an improved design which better integrates the requirements for angled fuel injector discharge thrust, fuel-air mixing, and burning stability without requiring increased fuel cooling.